LeetCode [41] First Missing Positive

 41. First Missing Positive

Hard

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3

Example 2:

Input: [3,4,-1,1]
Output: 2

Example 3:

Input: [7,8,9,11,12]
Output: 1

Follow up:

Your algorithm should run in O(n) time and uses constant extra space.

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class Solution { public int firstMissingPositive(int[] nums) { int n = nums.length; for(int i=0; i<n; ++i){ int v = nums[i]; int index = v-1; //move v to position index such that nums[index]==index+1 while(v>=1 && v<=n && nums[index]!=index+1){ int t = nums[index]; nums[index] = v; nums[i] = t; //move t to the right position if possible v = nums[i]; index = v-1; } } for(int i=0; i<n; ++i){ if(nums[i]!=i+1) return i+1; } return n+1; } }